[dockerfile] Add coreutils for command timeout.

The timeout command from coreutils and busybox exhibit different behaviors. Notably, busybox timeout may not reliably report the timeout status of a `docker service update` in some cases, leading to potential inaccuracies.

Dockerfile

@@ -9,7 +9,9 @@ RUN mkdir -p /gantry
 
 WORKDIR /gantry
 
-RUN apk add --update --no-cache curl tzdata docker-cli docker-cli-buildx
+# Add coreutils for command `timeout`.
+# The timeout command from coreutils and busybox exhibit different behaviors. Notably, busybox timeout may not reliably report the timeout status of a `docker service update` in some cases, leading to potential inaccuracies.
+RUN apk add --update --no-cache curl tzdata coreutils docker-cli docker-cli-buildx
 
 COPY src/* /gantry